perf/safety/docs improvements

src/Common/ConcurrencyControl.h

@@ -7,6 +7,16 @@
 #include <list>
 #include <condition_variable>
 
+#include <Common/Exception.h>
+
+namespace DB
+{
+namespace ErrorCodes
+{
+    extern const int LOGICAL_ERROR;
+}
+}
+
 /*
  * Controls how many threads can be allocated for a query (or another activity).
  * There is a limited amount of slots for threads. It can be set with `setMaxConcurrency(limit)`.
@@ -66,31 +76,42 @@ public:
     {
         ~Allocation()
         {
-            parent.free(this); // We have to lock parent's mutex to avoid race with grant()
+            // We have to lock parent's mutex to avoid race with grant()
+            // NOTE: shortcut can be added, but it requires Allocation::mutex lock even to check if shortcut is possible
+            parent.free(this);
         }
 
-        // Take one already granted slot if available
+        // Take one already granted slot if available. Lock-free iff there is no granted slot.
         [[nodiscard]] SlotPtr tryAcquire()
+        {
+            SlotCount value = granted.load();
+            while (value)
+            {
+                if (granted.compare_exchange_strong(value, value - 1))
                 {
                     std::unique_lock lock{mutex};
-            if (!granted)
-                return {};
-            granted--;
-            return SlotPtr(new Slot(shared_from_this()));
+                    return SlotPtr(new Slot(shared_from_this())); // can't use std::make_shared due to private ctor
+                }
+            }
+            return {}; // avoid unnecessary locking
         }
 
     private:
         friend struct Slot; // for release()
         friend class ConcurrencyControl; // for grant(), free() and ctor
 
-        Allocation(ConcurrencyControl & parent_, SlotCount limit_, SlotCount granted_)
+        Allocation(ConcurrencyControl & parent_, SlotCount limit_, SlotCount granted_, Waiters::iterator waiter_ = {})
             : parent(parent_)
             , limit(limit_)
             , allocated(granted_)
             , granted(granted_)
-        {}
+            , waiter(waiter_)
+        {
+            if (allocated < limit)
+                *waiter = this;
+        }
 
-        auto free()
+        auto cancel()
         {
             std::unique_lock lock{mutex};
             return std::pair{allocated - released,
@@ -99,11 +120,6 @@ public:
                     std::optional<Waiters::iterator>()};
         }
 
-        void wait(Waiters::iterator waiter_)
-        {
-            waiter = waiter_;
-        }
-
         // Grant single slot to allocation, returns true iff more slot(s) are required
         bool grant()
         {
@@ -119,18 +135,20 @@ public:
             parent.release(1);
             std::unique_lock lock{mutex};
             released++;
-            assert(released <= allocated);
+            if (released > allocated)
+                abort();
         }
 
         ConcurrencyControl & parent;
         const SlotCount limit;
 
         std::mutex mutex; // the following values must be accessed under this mutex
-        SlotCount allocated = 0; // allocated total (including already released)
-        SlotCount granted = 0; // allocated, but not yet acquired
+        SlotCount allocated; // allocated total (including already `released`)
         SlotCount released = 0;
 
-        Waiters::iterator waiter; // iterator to itself in Waiters list; valid iff allocated < limit
+        std::atomic<SlotCount> granted; // allocated, but not yet acquired
+
+        const Waiters::iterator waiter; // iterator to itself in Waiters list; valid iff allocated < limit
     };
 
 public:
@@ -142,26 +160,30 @@ public:
     // NOTE: Recommended way to achieve this is to use `instance()` and do graceful shutdown of queries
     ~ConcurrencyControl()
     {
-        assert(waiters.empty());
+        if (!waiters.empty())
+            abort();
     }
 
     // Allocate at least `min` and at most `max` slots.
     // If not all `max` slots were successfully allocated, a subscription for later allocation is created
-    // Use Allocation::tryAcquire() to acquire allocated slot, before running a thread.
+    // Use `Allocation::tryAcquire()` to acquire allocated slot, before running a thread.
     [[nodiscard]] AllocationPtr allocate(SlotCount min, SlotCount max)
     {
+        if (min > max)
+            throw DB::Exception("ConcurrencyControl: invalid allocation requirements", DB::ErrorCodes::LOGICAL_ERROR);
+
         std::unique_lock lock{mutex};
 
         // Acquire as much slots as we can, but not lower than `min`
-        SlotCount limit = std::max(min, max);
-        SlotCount granted = std::max(min, std::min(limit, available(lock)));
+        SlotCount granted = std::max(min, std::min(max, available(lock)));
         cur_concurrency += granted;
 
         // Create allocation and start waiting if more slots are required
-        auto allocation = new Allocation(*this, limit, granted);
-        if (granted < limit)
-            allocation->wait(waiters.insert(cur_waiter, allocation));
-        return AllocationPtr(allocation);
+        if (granted < max)
+            return AllocationPtr(new Allocation(*this, max, granted,
+                waiters.insert(cur_waiter, nullptr /* pointer is set by Allocation ctor */)));
+        else
+            return AllocationPtr(new Allocation(*this, max, granted));
     }
 
     void setMaxConcurrency(SlotCount value)
@@ -182,8 +204,13 @@ private:
 
     void free(Allocation * allocation)
     {
+        // Allocation is allowed to be canceled even if there are:
+        //  - `amount`: granted slots (acquired slots are not possible, because Slot holds AllocationPtr)
+        //  - `waiter`: active waiting for more slots to be allocated
+        // Thus Allocation destruction may require the following lock, to avoid race conditions
         std::unique_lock lock{mutex};
-        auto [amount, waiter] = allocation->free();
+        auto [amount, waiter] = allocation->cancel();
+
         cur_concurrency -= amount;
         if (waiter)
         {